The present invention relates to a magnetic resonance imaging (MRI) system.
In the MRI system, a magnetic resonance (MR) signal of an object under inspection is detected by making use of a magnetic resonance (MR) phenomenon. The detected data is subjected to an image reconstruction process by computed tomography (CT), and then programmed into a computed tomogram as a density distribution of specific atomic nuclei and a relaxation time constant distribution of a specific cross section of the object. The MRI system was initially developed for diagnosis in the medical field, but recently its application for nondestructive inspection has been studied.
In order to obtain a tomogram at a specific position of an object in the MRI system, an extremely uniform static magnetic field Ho is applied to an object under inspection, e.g., a human body P, in the direction of a z axis in FIG. 1. A linear magnetic gradient field Gz is superposed on the static magnetic field Ho by a pair of inclined magnetic field coils Za and Zb. In the gradient field Gz, the direction of the magnetic field is directed along the z axis, and its intensity is gradually increased or decreased along the z axis in a linear fashion. Under this condition, there are a number of x-y isomagnetic planes in parallel to each other and perpendicular to the z axis.
The atomic nucleus resonates with the static magnetic field Ho at an angular frequency .omega.o as given below EQU .omega.o=.gamma.Ho (1)
where .gamma. is a gyromagnetic ratio peculiar to the atomic nucleus, and its value depends on the type of atomic nucleus under study.
As mentioned above, when the human body P is under the static magnetic field Ho and the magnetic gradient field Gz, a pair of transmitting coils Ta and Tb applies (to the human body P) a rotating magnetic field H1 at an angular frequency wo to resonate only a specific atomic nucleus, viz., the angular frequency .omega.o for the specific atomic nucleus.
When the human body P is subjected to such magnetic fields, the MR phenomenon is caused only in a plane selected as an isomagnetic plane with a specific intensity, by the magnetic gradient field Gz, for example, an x-y plane in FIG. 1. The plane is a cross section which is used to obtain a tomogram of a slice S (which actually has a given thickness).
The MR phenomenon is observed as a free induction decay (FID) signal, through the receiving coils Ra and Rb. This signal is then Fourier transformed to have a single spectrum on the angular frequency .omega.o of the specific atomic nucleus. To reconstruct the tomograph as a computed tomogram, it is necessary to have projection information in as many directions in the x-y plane as in the slice S.
To obtain the projection information, after the slice S is excited to cause the MR phenomenon, a magnetic gradient field Gxy with a linear slope of the magnetic field along an x' axis (rotated by angle .theta. from the x axis on the x-y plane), as shown in FIG. 2, is superposed onto the static magnetic field Ho. As shown, the magnetic gradient field Gxy is such that its direction is in parallel with the z axis and its intensity gradually increases or decreases in the x' direction in a linear fashion. The magnetic gradient field Gxy causes linear and equal magnetic lines E1 to En with different values to be formed in the slice S of the human body P. The rotating angular frequencies .omega.o, of the nuclear spin of the specific atomic nuclei on the isomagnetic lines E1 to En, take values dependent on the intensities of the magnetic fields, which are each defined by the formula (1). Signals D1 to Dn as FID signals must be generated by the magnetic fields on the isomagnetic lines E1 to En. The amplitudes of the signals D1 to Dn are proportional to the density of the nuclear spins of the specific atomic nuclei, viz., the density of the specific atomic nuclei, on the isomagnetic lines E1 to En passing through the slice S. Actually, these signals D1 to Dn are not observed individually, but in the form of a composite FID signal Fd as the sum of these signals D1 to Dn. The projection data (one dimensional image) Pd to the x' axis of the slice S is obtained by Fourier transforming the composite signal Fd. By successively rotating the x' axis in the x-y plane (by changing .theta.), the projection data is collected for each predetermined angle, thereby obtaining projection data in each direction of the x-y plane. The projection data is used for the image reconstruction, to form a computed tomogram.
The rotation of the magnetic gradient field Gxy, that is, the change of .theta., will be described.
The magnetic gradient fields Gx and Gy in the x and y directions, respectively are formed by pairs of magnetic gradient field coils. These magnetic gradient fields Gx and Gy are combined to form a composite magnetic gradient field Gxy. One of the two pairs of magnetic gradient field coils is controlled to change the ratio of the magnetic gradient field Gx to that of the magnetic gradient field Gy. Then, the inclining direction of the composite magnetic field Gxy is changed.
When the human body P is placed between the receiving coils Ra and Rb, a quality factor (Q) of each of these coils is changed, as a matter of course. The change in Q depends on features of the human body P, for example, adult or child, fat or skinny, man or woman, the physical configuration of the human body P, large or small, etc. As a result, it is equivalent to the change in the gain of the receiving system.
When Q is changed, the density of specific atomic nuclei on the image (e.g., a proton density) inevitably changes depending on the differentials of the human body P. This fact rejects a quantitative evaluation of the MR image data.